Some vehicle electronic apparatuses require an alternating-current voltage. For example, a liquid crystal display apparatus for a vehicle requires a light source for backlighting to obtain bright images on a liquid crystal panel. For the light source for backlighting, usually a discharge lamp (e.g., a fluorescent lamp) is used.
The fluorescent lamp is lit by a high-voltage high-frequency alternating-current voltage of an inverter, which converts the direct-current voltage of a battery into the high-frequency alternating-current voltage. Then, the alternating-current voltage is boosted by a power transformer and applied to the fluorescent lamp (see U.S. Pat. No. 5,384,516, corresponding to Unexamined Japanese Patent Publication No. H05-242987).
The inverter includes a plurality of power switching elements, which are connected in the bridge form. The liquid crystal display apparatus, which is mounted in the car, is often exposed to very severe conditions. In the conditions, the switching elements may suffer from an internal defect, and may generate heat.
Conventionally, the output frequency of the inverter is detected by a microcomputer in order to detect defects. When the output frequency changes, a power supply to the inverter is stopped.
The switching elements are soldered to a circuit board. Solder sometimes becomes to have a large resistance. This arises if the soldering is not properly performed and bubbles are captured in the solder, or if a soldering surface of a terminal is only partly soldered. When the solder has the large resistance, the solder generates heat. Therefore, the temperature of the switching elements increases, and this may result in causing defects.
However, for this kind of soldering defects of the switching elements, the conventional defect detective method, which detects a change in the output frequency of the inverter, cannot detect the defects of a soldering part. This is because the output frequency does not change, even when the temperature of the soldering part increases.